meson: build test-vaapicontext when using X11

meson: build test-vaapicontext when using X11

x11_dep and libva_x11_dep are optional and meson ignores these
dependencies even if they are added into the dependency list.

This fixes the error below when libva-x11 is not avaiblabe:

cc -Itests/elements/tests@elements@@test-vaapicontext@exe
-Itests/elements -I../../gstreamer-vaapi/tests/elements -I.
-I../../gstreamer-vaapi/ -Igst-libs -I../../gstreamer-vaapi/gst-libs
-I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/orc-0.4
-I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0
-I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include
-I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0/
-I/usr/include/libdrm -I/usr/include/harfbuzz -I/usr/include/pango-1.0
-I/usr/include/fribidi -I/usr/include/atk-1.0 -I/usr/include/cairo
-I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2
-I/usr/include/libpng16 -I/usr/include/gdk-pixbuf-2.0
-fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall
-Winvalid-pch -O2 -g -fvisibility=hidden -fno-strict-aliasing -pthread
-DHAVE_CONFIG_H  -MD -MQ
'tests/elements/tests@elements@@test-vaapicontext@exe/test-vaapicontext.c.o'
-MF
'tests/elements/tests@elements@@test-vaapicontext@exe/test-vaapicontext.c.o.d'
-o
'tests/elements/tests@elements@@test-vaapicontext@exe/test-vaapicontext.c.o'
-c ../../gstreamer-vaapi/tests/elements/test-vaapicontext.c
../../gstreamer-vaapi/tests/elements/test-vaapicontext.c:29:10: fatal
error: va/va_x11.h: No such file or directory
 #include <va/va_x11.h>